Chemical sensing trench field effect transistor

ABSTRACT

A field effect transistor (10) for chemical sensing by measuring a change in a surface potential of a gate electrode (48) due to exposure to a fluid has a semiconductor substrate (12) with a trench (18, 20). The trench has a first sidewall (30) and a second sidewall (32) disposed opposite the first sidewall to provide a fluid gap (50) for the fluid to be sensed. The gate electrode is disposed overlying the first sidewall of the trench, and a source region (54) and a drain region (56) are disposed in the second sidewall of the trench. A channel region (52) is disposed between the source and drain regions, and the gate electrode is disposed opposite the first channel region across the fluid gap. A heater (26) for regulating the temperature of the gate electrode is disposed in the first sidewall of the trench.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to semiconductor devices and,more particularly, to a field effect transistor for measuring a changein surface potential of the transistor's gate electrode due to exposureto a chemical as used in, for example, a chemical sensor.

Field effect transistors have been previously used in some cases aschemical sensors for measuring the concentration of a chemical in afluid. One such prior sensor uses a gate electrode that is horizontallysuspended over the channel region so as to provide a gap in which fluidmay enter and contact an exposed surface of the gate electrode. Achemical in the fluid, to which the gate electrode is particularlysensitive, is adsorbed onto the exposed surface and changes the surfacepotential of the gate electrode. The drain current of the transistorchanges in response to this surface potential change. Thus, if aconstant gate voltage source is applied to the gate electrode duringsensing, the change in drain current can be correlated to theconcentration of the chemical in the fluid.

It has been found that the surface chemical reactions of this priorsensor, which include adsorption/desorption reactions of the chemical tobe sensed onto and off of the exposed gate electrode surface, are verysensitive to temperature, so it is desirable that the temperature of thegate electrode be more directly regulated to optimize the output of thesensor. Also, it has been found to be desirable that this temperature beelevated above the ambient temperature to provide improved performancefor the sensor. However, prior chemical sensors do not provide anintegrated heating element for direct temperature control of the gateelectrode. Instead, an external heater is required to heat the entiresensor assembly, rather than the gate electrode directly. Such anexternal heater is inconvenient to provide in a final,fully-manufactured chemical sensor assembly and increases themanufacturing cost thereof. Also, an external heater requiressignificant power consumption during operation.

In addition to the above, it is important that the gap size of thesensing transistor be readily controllable during manufacture so thatthe operating characteristics of the transistor are consistent over alarge number of manufactured sensors. Also, it is important that the gapstructure be stable under thermal loads, and the gap structure should bemechanically robust to improve reliability along with ease of handlingand packaging.

Further, the mechanical structure of the transistor should permit readydiffusion of the fluid to be sensed into and out of the gap of thetransistor. This ready diffusion is important to provide a fasterresponse time of the sensor to chemical changes in the fluid. Thesechanges cannot be fully sensed until the species corresponding to thechemical change diffuses through the gap of the transistor structure tocontact the gate electrode of the sensor. Prior sensor structures do notprovide a readily controllable gap size and such ready diffusion intothe gap. Further, prior sensor structures need to be more compatible forintegration into standard device process flows for devices for controlcircuitry incorporated on the same chip as the sensor.

Accordingly, there is a need for a chemical sensing field effecttransistor that provides an integrated heater and a fluid gap in atransistor structure that permits improved control of the gap size,improved diffusion of a fluid into the gap, and improved compatibilitywith other standard device process flows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 are enlarged cross-sectional views illustrating sequentialsteps in the manufacture of a chemical sensing field effect transistoraccording to the present invention; and

FIG. 6 is a top schematic view of the chemical sensing field effecttransistor of FIG. 5

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention generally provides a chemical sensingsemiconductor device having a trench and provides in one specificembodiment a trench field effect transistor for measuring a change inthe surface potential of the transistor's gate electrode due to exposureof the electrode to a fluid. As used herein, the term "fluid" includesboth gaseous and liquid fluids. Typically, the gate electrode is a metalonto which chemicals to be sensed adsorb. The adsorption causes a changein the surface potential of the metal that then modulates theconductivity of the channel region of the transistor. The gate electrodeis disposed opposite the channel region across a gap containing thefluid to be sensed. By changing the material used for the gateelectrode, the sensitivity of the transistor can be tailored to any of abroad spectrum of different chemicals at varying ranges ofconcentration.

FIGS. 1-5 are cross-sectional views illustrating sequential steps, forone specific embodiment, of the manufacture of a chemical sensing fieldeffect transistor 10 according to the present invention. As will becomeapparent later, transistor 10 includes a plurality of devices connectedin parallel. It is should be noted that FIGS. 1-5 are not drawn to scaleand are exaggerated for purposes of illustration. FIG. 1 illustrates asemiconductor substrate 12 such as, for example, a P- silicon wafer. Adoped region 14 of a conductivity type different from substrate 12 hasbeen formed in substrate 12 by, for example, a high dose N+ implant ofarsenic or phosphorous of about 1E15-1E16/cm². This implant has beenthermally driven in to a depth of, for example, about 0.5 to 10 micronsfrom a top surface 13 of substrate 12. The depth of doped region 14, aswill be seen later, corresponds to the channel width of each individualdevice to be formed in transistor 10.

A hard mask 16 has been formed and patterned on top surface 13 ofsubstrate 12. Hard mask 16 is, for example, an oxide layer formed byplasma enhanced chemical vapor deposition (PECVD). Openings are formedin hard mask 16 by a conventional pattern and etch process andcorrespond to the position of trenches to be formed later in substrate12.

FIG. 2 illustrates substrate 12 after trenches 18 and 20 have beenformed therein using a conventional anisotropic trench etchingtechnique. For example, trenches 18 and 20 can be etched using areactive ion etch in a standard chlorine or fluorine chemistry. Thewidth of each trench has a minimum dimension of, for example, about 0.05to 20 microns. Trenches 18 and 20 extend to a depth past the lowerboundary of doped region 14 (see FIG. 1) so as to divide doped region 14into portions corresponding to a drain 22, a source 24 and a heater 26.As will be discussed in more detail later, in a preferred embodimentdrain 22, source 24, and heater 26 will have the same dopantconcentration since they are formed in a single implant step from commondoped regions 14. Alternatively, heater 26 may have a higher dopantconcentration by the use of an additional, separate implanting step forthe region of substrate 12 corresponding to heater 26.

It should be noted that hard mask 16 is somewhat reduced in thicknessfrom the etching of trenches 18 and 20. Also, a ridge 28 having a topsurface 29 is disposed between trenches 18 and 20 and corresponds to acontinuous doped region that provides heater 26 (see FIG. 6).

Following the etching of trenches 18 and 20, a sidewall oxide layer 34is formed, for example, using a PECVD oxide. Sidewall oxide layer 34protects ridge sidewall 30 and outer sidewall 32 of each trench duringsubsequent etching steps.

One advantage of the present invention is that the width of each trenchis determined by a photolithography patterning step rather than by theuse of sacrificial layers or stand-off structures. This provides easierand more uniform control of each trenches width from one sensor chip toanother. Further, as shown later, the trench width largely determinesthe size of the gap into which fluid enters for sensing (see fluid gap50 of FIG. 5).

FIG. 3 illustrates the additional etching of trenches 18 and 20. First,in order to perform this additional etching, the portions of sidewalloxide layer 34 on the bottoms of each trench are removed using, forexample, a conventional dry anisotropic oxide etch. For example, CF₄ ata low pressure can be used. It should be noted that this etching stepalso substantially removes the top portions of sidewall oxide layer 34overlying hard mask 16.

After removing these bottom portions of layer 34, a cavity 36 and acavity 38 are formed by a two-step etching process. First, ananisotropic etch using similar chemistry as the prior anisotropic etchabove is used to extend the depth of each trench. The depth to whicheach trench is extended is determined in part by the desire toelectrically isolate a gate electrode 48 (see FIG. 5), which will beformed later over ridge 28, from the source and drain regions of thefinal sensor. For example, the depth may be extended by up to about 50percent.

Second, an isotropic etch is performed using, for example, SF₆ toprovide an undercut 39 in each trench. As will be seen later, after gateelectrode 48 (see FIG. 5) is later formed over ridge 28, undercut 39will assist in electrically isolating any stray gate electrode formationmaterial that might remain on outer sidewall 32 overlying source ordrain 22 or 24. Undercut 39 widens each cavity by, for example, about anadditional 4-6 microns from the vertical sidewalls of each trench (i.e.the cavity width is greater than the upper trench width by about 4-6microns). Undercut 39 in each of cavities 36 and 38 makes themanufacturing process more robust and facilitates later processingassociated with the formation of the gate electrode by isolating thegate electrode from any stray gate electrode material in this way. Someof the factors that affect the degree of undercut 39 needed include thegate electrode thickness, the thickness of the later-formed gate oxidelayer 40 (see FIG. 5), the size of fluid gap 50 (see FIG. 5), and thedepth of each trench from the wafer surface to the cavity bottom.Following the formation of cavities 36 and 38, all remaining portions ofsidewall oxide layer 34 and hard mask 16 are removed, for example, usinga wet etch of HF.

In FIG. 4, gate oxide layer 40 has been formed overlying drain 22,source 24, and heater 26 by, for example, thermally growing a siliconoxide layer to a thickness of about 400-5,000 angstroms. This is done inpreparation for gate electrode 48.

Now, referring to FIG. 5, source and drain contacts are opened and afirst metal layer is formed using conventional techniques to provide asource electrode 42 and a drain electrode 44. These electrodes areformed, for example, using an aluminum alloy. It is preferable that thepatterning of the first metal layer be done using a wet etch, but a dryetch is also acceptable. Next, a passivation layer 46 is formedoverlying gate oxide layer 40 and overlying source and drain electrodes42 and 44. Passivation layer 46 is, for example, a conformal siliconnitride layer formed by chemical vapor deposition to a thickness ofabout 1,500-7,000 angstroms. Also, although not illustrated in thefigures, contact openings are formed to heater 26 on each of its ends(see FIG. 6 for a top view of heater 26).

The final processing step illustrated in FIG. 5 is the formation of gateelectrode 48 which provides an adsorption layer for chemical adsorptionfrom a fluid present in fluid gap 50. Gate electrode 48 is a continuouslayer formed overlying top surface 29 of ridge 28 and is generallyconformal to the topography of underlying passivation layer 46. Thematerial used to form gate electrode 48 is selected depending on theparticular chemical sensing application desired. For example, gateelectrode 48 may be formed of a metal such as platinum, palladium, oralloys thereof. Fluid gap 50 permits fluid to contact gate electrode 48.

In the case of a metal gate electrode, gate electrode 48 can be formedby sputtering the metal into trenches 18 and 20 and over top surface 29.Next, the sputtered metal is patterned using a conventional thickphotoresist and patterning chemistry. The thickness of gate electrode 48will vary depending on the particular material selected and the sensingapplication, but typically this thickness varies between about 50-7,000angstroms. Although sputtering is described here, it should also beappreciated that gate electrode 48 can also be deposited by otherconventional processes including chemical vapor deposition.

FIG. 6 is a top schematic view of transistor 10. It should be noted thatfor purposes of illustration FIG. 6 does not show gate electrode 48,passivation layer 46, source and drain electrodes 42 and 44, or gateoxide layer 40. In addition to drain 22 and source 24, transistor 10 inthis illustrated embodiment includes a plurality of sources 54 anddrains 56 connected in parallel. A plurality of channel regions 52 areeach disposed on outer sidewalls 32 of trenches 18 and 20. Each channelregion 52 carries a current 58 that flows laterally along outer sidewall32. It should be noted that this current does not flow from a source onone side of ridge 28 to a drain on the opposite side of ridge 28 byflowing underneath ridge 28.

Chemical sensing field effect transistor 10 has fluid gap 50 (see FIG.5) between gate electrode 48 and the source and drain regions oftransistors 10, which include drain 22 and source 24. In operation, afluid enters fluid gap 50 and is adsorbed onto gate electrode 48. Theadsorption of chemicals in the fluid onto gate electrode 48 changes thesurface potential of the electrode and in turn modulates theconductivity of the corresponding channel region 52 (see FIG. 6) acrossfluid gap 50 on the opposite side of the respective trench. Anotherexample of a chemical field effect transistor that uses chemicalsensitivity of a gate electrode to chemical exposure is described incommonly-assigned U.S. application Ser. No. 08/427,389, filed on Apr.24, 1995, by Young S. Chung and titled "CHEMICAL PROBE FIELD EFFECTTRANSISTOR FOR MEASURING THE SURFACE POTENTIAL OF A GATE ELECTRODE INRESPONSE TO CHEMICAL EXPOSURE," which is hereby incorporated byreference in full. It should be appreciated that transistor 10 accordingto the present invention provides fluid gap 50 in a trench 18 or 20 thatmore readily permits diffusion of fluid into and out of the trench sothat the response time of transistor 10 to changes in chemicalconcentrations is much greater compared to prior types of chemicalsensing transistor structures.

The plurality of devices of transistor 10 are connected in parallel suchthat a common drain voltage bias is applied to each drain by a terminal60, and a common source voltage bias is applied to each source by aterminal 62. Gate electrode 48 will be biased by a contact (not shown)on one of its ends to maintain the bias current necessary to achieve auseful signal-to-noise ratio from transistor 10. Heater 26 is used tocontrol the temperature of gate electrode 48 and thereby regulate theabsorption/desorption rate of chemicals in fluid gap 50 onto and fromgate electrode 48. Heater 26 will be biased by contacts (not shown) onboth of its ends to heat gate electrode 48 to a temperature optimal forthese adsorption/desorption reactions at the gate electrode's surface.Also, heater 26 is electrically controllable independently of gateelectrode 48. Transistor 10 is preferably operated in a depletion mode,having conducting channel regions 52 in the presence of little or nobias voltage on gate electrode 48. The channel length for each device isvariable depending upon the particular application, but typically variesbetween about 0.05 to 20 microns. The overall length of each trench 18or 20 depends on the particular device parameters, but can range forexample between about 100 microns to 10 millimeters. The operatingtemperature range for transistor 10 also depends on the application, buta typical temperature range is from about 100°-550° K. Further, it isnot necessary that two trenches be used. Instead, a single trench ormultiple trenches having many different arbitrary geometricalconfigurations can be used.

As mentioned previously, the channel width of each device substantiallycorresponds to the depth of the corresponding doped region 14 (seeFIG. 1) into substrate 12. With the devices of transistor 10 connectedin such a parallel configuration, the overall width/length (W/L) ratiofor transistor 10 is given by W(n-1)/L, where W is the depth of dopedregion 14, n is the number of source and drain regions, and L is thechannel length. As the length of each trench is increased, additionalsource/drain diffusions can be added so that a longer trench willprovide a higher overall W/L ratio.

Chemical sensing is accomplished by measuring changes in the overalldrain current of transistor 10, for example, at terminal 60. Because thedrain current is directly proportional to the overall W/L of transistor10, the length of each trench is selected to provide a drain current ofa sufficient magnitude for measurement by the control electronics on thesensor.

As discussed previously, doped region 14 of FIG. 1 is used to providedrain 22 and source 24 after the etching of trenches 18 and 20 (see FIG.5). As is illustrated in FIG. 6, transistor 10 has a plurality ofsources 54 and drains 56. Each source and drain pair in this specificembodiment is disposed symmetrically across heater 26 and corresponds toa doped region formed earlier in the processing sequence such as dopedregion 14 of FIG. 1. In a preferred approach, sources and drains 54 and56 along with heater 26 are formed by a single implant mask patternincluding several adjacent doped regions 14 connected by a spine-likeportion of the implant pattern corresponding to heater 26.

By now it should be appreciated that there has been provided a novelchemical sensing transistor that integrates a heater into a devicestructure using a trench to define the size of the fluid gap. Thistrench structure provides improved diffusion of fluid into and out ofthe gap leading to improved response time of the sensing transistor tochanges in chemical concentration. Further, the trench structure oftransistor 10 makes it more manufacturable than prior sensors due to theimproved precision and accuracy associated with forming fluid gap 50.Transistor 10 is also more mechanically stable under thermal stressesinduced by heater 26 and the environment in which the final sensorpackage is used. This improved mechanical structure improves long-termdevice reliability, and ease of handling and testing throughqualification and delivery to an end user.

The chemical sensing transistor of the present invention is useful in awide number of applications including the sensing of gaseous hydrides,oxygenated organic vapors, sulfur compounds, and other gaseous or liquidspecies that cause a reversible work function change of the gateelectrode.

The foregoing discussion discloses and describes merely exemplarymethods and embodiments of the present invention. As will be understoodby those familiar with the art, the invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting, of the scopeof the invention, which is set forth in the following claims.

We claim:
 1. A field effect transistor for measuring a change in asurface potential of a gate electrode due to exposure to a fluid, saidfield effect transistor comprising:a semiconductor substrate having afirst trench and a second trench disposed on opposite sides of a ridgeprotruding upward between said first trench and said second trench,wherein said first trench has a first fluid gap, said second trench hasa second fluid gap, and each of said first trench and said second trenchhas a ridge sidewall disposed on said ridge and an outer sidewalldisposed opposite said ridge sidewall; a first source region and a firstdrain region disposed on said outer sidewall of said first trench; and asecond source region and a second drain region disposed on said outersidewall of said second trench, wherein said gate electrode has a firstportion disposed overlying said ridge sidewall of said first trench anda second portion overlying said ridge sidewall of said second trench. 2.The field effect transistor of claim 1 wherein said gate electrode is acontinuous layer including said first port ion and said second portionand further overlying a top surface of said ridge.
 3. The field effecttransistor of claim 2 further comprising a heater disposed in saidridge.
 4. The field effect transistor of claim 3 wherein said firstdrain region and said second source region are disposed in substantiallysymmetric positions across said ridge.
 5. The field effect transistor ofclaim 1 wherein said semiconductor substrate further comprises a firstcavity connected to and underlying said first trench and a second cavityconnected to and underlying said second trench.
 6. The field effecttransistor of claim 5 wherein said first source region and said secondsource region are coupled to a common source voltage bias, and saidfirst drain region and said second drain region are coupled to a commondrain voltage bias.